A series of random and irregular jottings, documenting matters of interest (to me) in the sphere of amateur radio and beyond.

Sunday, 1 July 2012

Teething Troubles with the Leslie

No sooner than I'd completed the modifications which turned my Leslie 125 into something approaching a 145, I ran into a problem...

Without warning, the amp popped its fuse. On investigation, it turned out to be the mains transformer. I could measure resistances of the order of tens of Ohms where there ought to be isolation - between the secondary, the primary and the core. Also, the secondary centre tap was no longer in the centre - in impedance, at least. Obviously, where once there was insulation there were now some conducting paths.

Maybe I've been lucky, but in 35+ years of tinkering with electronics, I never met a bad transformer before. They just work - or so I thought!

Just my luck - it is easy to find replacement transformers for a 122 or a 147 - they're stock items (especially in the US). But I couldn't find a source for a replacement for my 125 amp's transformer for love or money. Fortunately, Simon at BLS Electronics made me a new one, re-using the pressed steel shrouds from the old unit.

Here are the remains of the original, next to the new transformer...

Usual disclaimer - I don't know Simon or BLS Electronics from Adam - but they did a great job at a great price. The new transformer is now in place and everything was working fine until...

After having played though the newly restored speaker for best part of a day, I noticed that the lower rotor was creeping round in chorale. It would run in tremolo, but ground to a halt at the lower speed. I fiddled about and noticed that if I pushed up the rotor on its spindle (on which it is a friction fit), the rotor would turn again. But, after a few more hours, the old sluggish behaviour returned. Time to strip down the lower rotor...

Here's an exploded image of the rotor from the user's manual...

The drawing indicates that the bearing plate (#511-2 in the drawing above) is fixed in place from beneath the cabinet by two screws. I tilted the speaker enough to get a short screwdriver to the screws, but they didn't have any heads!

Instead, I saw the ends of the screws in some T-nuts.
Despite expectations, the bearing unit in my 125 is fixed in from inside, on spacer blocks...

From what I could see, this is how it was made - no evidence of modifications of any kind.
I could also quickly see what was slowing down the rotor - especially when it was "low" on the spindle. Here's the top of the bearing assembly, in the middle of which you see the grommet which goes through in the middle of the bearing (numbered 510-3 in the drawing from the user's manual)...

According to the user manual (see exploded drawing above) another grommet (513-2) at the bottom of the rotor bears on this grommet to support the weight of the rotor. Looking at the bottom of the rotor reveals that it had been scraping against the bearing housing, generating the drag that brought the whole assembly to a halt...

Also, I noticed that the bearing housing was distorted in such a way as to bring the bearing lower in the housing - adding to the possibility that the rotor grommet could foul on stationary parts. Look at the deformation in the lower housing component...

The fix was obvious - I needed to keep the rotor higher up the spindle, so the rubber grommet at the base was free. I took a "belt-and-braces" approach and made three interventions.

First, I cleaned and assembled the bearing, avoiding the distortion in the photo above, fractionally moving the ball race and its bearing surface higher.

Second, I inverted the grommet in the bearing, so its unworn lower part is now at the top...

Third, I added a washer between the grommet in the bearing and that on the rotor (between which there is no significant relative movement) to make double-sure there is real clearance between fixed and rotating parts.

The result? The speaker is transformed, with working chorale and much faster acceleration to tremolo. It has been running all day without a hint of trouble. I hope that has fixed it for good.

I do have some outstanding jobs, most important of which is to cure a rattle/buzz coming from part of the rotor - there's a flared piece (made, I think, of expanded polystyrene) which is moving relative to the other wooden parts and - in so moving - buzzing. I've put some Blu Tack there as a temporary fix.

Excuse me - I must go back to the organ now - this speaker sounds SO good.

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About Me

Engineer, who grew up in the age when hobbyists used a soldering iron (rather than a mouse) to mess with computers. Came late in life to amateur radio, where he operates the station m0xpd, conducts pointless experiments, and plays with words and ideas.